Glass carrier specifications

Discussion in 'Darkroom Developing and Printing' started by HypoBob, Jan 10, 2004.

  1. HypoBob

    HypoBob Guest

    What are the design criteria for making a glass negative carrier? The
    negative will be sitting on a piece of glass, which, if not coated, will
    reflect about 8% of the light back up to the negative and introduce
    scattered light. Likewise there will be a small amount of scattering
    between the top glass and the back of the negative.

    The light must then pass through the bottom glass with the least
    possible perturbation so that the lens can focus it as sharply as it
    could without the glass.

    It would seem that to be done well, the glass should be very thin,
    optically flat, and at least mono-coated. Are glass carriers indeed
    made this way? Does that account for their high prices. ($200 for two
    pieces of glass and two pieces of aluminum is approaching military
    procurement pricing.)

    Bob
     
    HypoBob, Jan 10, 2004
    #1
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  2. All I can tell you is what I'm using in my antique Elwood 5x7: two pieces of
    plain window glass, single-strength. Coating, schmoating.

    If you're concerned about flatness, get float glass. (I think Edmund Optical
    sells it for less than milspec prices.)

    Try it.
     
    David Nebenzahl, Jan 10, 2004
    #2
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  3. I forgot to say that it works fine for me (with 9x12 & 4x5 negs).
     
    David Nebenzahl, Jan 10, 2004
    #3
  4. Reflection and scattering are not the same. Reflection is specular with a
    predictable direction to the reflected rays. Scattering would normally have
    rays diverted to many directions.
    Coated would be a different way of avoiding Newton's rings but would cost even
    more. Clear, uniform glass won't perturb the rays inside of the glass. If the
    glass is of optically good quality, there is no need for thinness.
    Usually not. Typical practice is uncoated glass: regular glass for the bottom
    and usually anti-Newton's-rings glass for the top. The inner surface of the ANR
    glass is etched so as to prevent Newton's rings from appearing on the print.

    The price is from small production volume of a more more complicated carrier.
    It would cost even more if made to aerospace/military standards.

    --Michael
     
    Michael Briggs, Jan 10, 2004
    #4
  5. HypoBob

    lloyd Guest


    jan1004 from Lloyd Erlick

    How big a negative are you working with?

    The Durst glass inserts (for 4x5, two glasses, top
    and bottom) sold for about USD75 a few years ago,
    no doubt they are more now. I think a fine glass
    carrier could be made by clever fingers from a set
    of those. I actually use a set in a genyewine
    durst enlarger, and I must say I regret not using
    glass carriers earlier in my career. I poohpoohed
    them foolishly. As another poster to this group
    has said many times, there's no reason to align
    your enlarger if you're not going to use a glass
    carrier

    regards,
    --le.
    ______________________________
    Lloyd Erlick Portraits, Toronto.
     
    lloyd, Jan 10, 2004
    #5
  6. The surface reflection will be considerably less than
    this if the film and glass are in intimate contact. The
    amount of reflection depends on the difference in the index
    of refraction of the two media. Since both gelatin and film
    support have indexes much closer to glass than air the
    amount of reflection at the interfaces will be negligible.
    Good contact will also eliminate Newton's rings.
    Its important that the glass be uniform and free of
    bubbles or waves. Most modern float window glass is quite
    suitable. The glass must be thick enough so that it doesn't
    deform, probably 1/8th inch thick is about right.
    While low index optical glass would be ideal because it
    is clear and not green, the difference is really negligible
    and doesn't justify the rather high cost of the optical
    glass.
    While there is a theoretical effect on aberrations of the
    glass on the image side its of no practical importance.
     
    Richard Knoppow, Jan 10, 2004
    #6
  7. HypoBob

    Norman Worth Guest

    There are no specifications that I know of. The better carriers use special
    flat glass (flatter than ordinary float glass) for the bottom pane. The
    film emulsion contacts this pane evenly, and reflections are not a problem.
    The upper pane is often anti-Newton ring glass. It is also flatter than
    ordinary glass, but it has a slight pattern etched on the bottom side to
    prevent Newton ring interference and reflection problems when it is in
    contact with the base side of the film.
     
    Norman Worth, Jan 11, 2004
    #7
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